It is well known in the Nylon industry that ADN can be hydrogenated catalytically to produce HMD by complete hydrogenation, or mixtures of ACN and HMD by partial hydrogenation. HMD can be used in the manufacture of Nylon 6, 6. The hydrogenation reaction product also contains unreacted ADN and unwanted byproducts such as THA. After hydrogenation, the reaction product must be refined, generally by methods involving fractional distillation, and HMD and ACN must be separated from each other.
It is also known that if the refining conditions involve too high a temperature, the unreacted ADN can isomerize into CPI. The CPI generally distills with the ADN, and can form AMCPA (2-aminomethylcyclopentylamine) if it is recycled back to the hydrogenation reactor. AMCPA, if unseparated from the HMD, can cause inferior Nylon 6, 6 to be made.
U.S. Pat. Nos. 6,346,641 and 6,462,220 teach distillation processes to separate ACN and HMD in which the column temperatures are kept below 185 degrees C. However, neither of these patents teach methods that allow distillation to be performed in a manner in which HMD can be recovered substantially free of THA.
U.S. Pat. No. 6,300,497 B1 teaches a method for reducing the THA content of a THA/HMD mixture by distillation using column head pressures between 0.3 and 3.0 bar, as well as reducing the THA content of a THA/ACN mixture by distillation using column head pressures between 0.1 and 1.3 bar. U.S. Patent Application No. 2003 0023083 teaches a method for reducing the THA content of a THA/HMD mixture by distillation using column head pressures between 0.001 and 0.3 bar, as well as reducing the THA content of a THA/ACN mixture by distillation using column head pressures between 0.001 and 0.2 bar. However, neither of these disclosures teaches a method in which a three component ACN/HMD/THA mixture is distilled so that the ACN and the HMD can be separated from one another in such a way that a substantial portion of the THA remains with the ACN, particularly when the three component ACN/HMD/THA mixture is one that is derived from the product that is produced by the partial hydrogenation of ADN, such a product containing unreacted ADN that is capable of being isomerized into undesirable CPI if distillation temperatures in the refining train exceed about 195 degrees C.
U.S. patent application Ser. No. 10/383,947 discloses a method for recovering HMD and ACN from a mixture comprising HMD, ACN, THA, and ADN, the method comprising the steps (a) and (b) below:
(a) introducing a mixture comprising HMD, ACN, THA, and ADN into a first distillation column; separating as a group the HMD, ACN and at least a portion of the THA from the ADN, while minimizing isomerization of the ADN into CPI; and then
(b) introducing the stream of HMD, ACN and the THA into a second distillation column and separating a distillate comprising HMD from column tails comprising ACN under conditions so that the THA separates along with the ACN in the tails.
Step (b) is accomplished preferably under column conditions of head pressure of at least about 200 mm Hg absolute and a pressure drop across the column of greater than about 25 mm Hg. Under these column conditions, however, there is significant formation of dimers of ACN and HMD as well as mixed dimers of ACN and HMD that separate along with the ACN and the THA in the column tails. Presence of these dimers in the ACN and THA is believed to be detrimental to catalyst life in the process of making caprolactam from the column tails.
It would, therefore, be desirable to have a method of producing a mixture comprising ACN and THA that is substantially free of dimers of ACN and HMD as well as mixed dimers of ACN and HMD for use in making caprolactam without greatly affecting catalyst life. The present invention provides such a method.